Field of the Invention
This invention relates to a method of manufacturing an element with a multiple-level step-like structure such as a diffractive optical element, a Fresnel lens, a phase type computer hologram (CGH), or a mask for the CGH, for example, more particularly, an element having a very fine surface-step pattern such as a diffractive optical element, for example, usable in manufacture of a semiconductor integrated circuit, for example. In another aspect, the invention is concerned with a method of manufacturing a mold for producing such an element.
Fujita, et al. (xe2x80x9cJournal of Electronic Communication Associationxe2x80x9d, (C) J66-CP85-91, January, 1983) and Japanese Laid-Open Patent Application, Laid-Open No. 26560/1987 as well as Japanese Laid-Open Patent Application, Laid-Open No. 42102/1987 disclose a method wherein a step-like shape is formed by using an electron beam while controlling its dose quantity, and wherein a resist is used directly as a circuit pattern.
Japanese Laid-Open Patent Application, Laid-Open No. 137101/1986 discloses a method wherein two or more types of films having an etching durability are accumulated with a desired thickness, wherein the layers are etched from the top layer to provide a step-like structure, whereby a mold is formed.
Japanese Laid-Open Patent Application, Laid-Open No. 44628/1986 and Japanese Laid-Open Patent Application, Laid-Open No. 160610/1994 disclose a method wherein, while a resist is used as an etching mask, a step-like structure is formed on the basis of a sequential alignment for the steps, whereby a mold is formed.
Japanese Laid-Open Patent Application, Laid-Open No. 15510/1996 discloses a method wherein an etching stopper layer is used and, for each step, an etching stopper layer and a transparent layer are accumulated, and wherein a step-like structure is formed through alignment, exposure and etching.
Japanese Laid-Open Patent Application, No. 26339/1994 which corresponds to Japanese Laid-Open Patent Application, Laid-Open No. 72319/1995 and U.S. Pat. No. 5,324,600 disclose a method wherein an alignment operation is performed while using a resist as an etching mask, and a step-like structure is formed.
In these examples, however, the minimum size is determined by the smallest resolution of a drawing apparatus and, therefore, it is not easy to produce a very fine shape.
A multiple-level step-like diffractive optical element can be manufactured as a diffractive optical element having a step-like sectional shape, in accordance with photolithographic processes based on exposure and etching techniques that are used in semiconductor manufacture. In such a multiple-level step-like diffractive optical element, the function of a diffractive optical element is performed by step-like surface level differences (steps) formed on a substrate.
Therefore, the optical performance of the multiple-level step-like diffractive optical element, particularly, the diffraction efficiency, depends on the shape of the formed surface step, that is, depth, width, or sectional shape of the step, for example. Specifically, where plural masks are used, an alignment error between them largely influences the diffraction efficiency.
For example, where a step-like shape is to be formed by using masks of harmonic frequencies sequentially, an idealistic step-like shape can be formed if there is no alignment error or line width error. Practically, however, it is very difficult to remove the line width error or the alignment error completely and, therefore, the produced shape differs from the idealistic shape. Basically the same problem is involved in other methods.
Referring to a specific example, as shown in FIG. 70, idealistically an eight-level (step) shape can be produced by using three masks, that is, masks A, B and C. If any misregistration occurs between the masks A and B, a problem arises. FIG. 70 shows the resultant shape when there are alignment deviations db and dc among the masks A, B and C. If the illustrated shape results, the optical performance of the optical element D such as diffraction efficiency, for example, considerably degrades.
Also, if there is a line width error at each layer, the optical performance such as diffraction efficiency further degrades. In the case of electron beam drawing, there may be no alignment error. However, a bulky drawing operation is required and, therefore, a sufficient throughput with respect to the productivity is not attainable.
It is an object of the present invention to provide an improved method of manufacturing an element having an accurate and very fine step-like shape.
It is another object of the present invention to provide a method of manufacturing a mode for producing an element having an accurate and very fine step-like shape.
In accordance with an aspect of the present invention, there is provided a method of manufacturing an element having a multiple-level step-like shape through plural lithographic processes, wherein a position of at least one step of the step-like shape is determined by an end of at least a portion of a pattern of a first mask to be formed through a first lithographic process of the plural lithographic processes.
In accordance with another aspect of the present invention, there is provided a method of manufacturing a mold for production of an element having a multiple-level step-like shape through plural lithographic processes, wherein a position of at least one step of the step-like shape is determined by an end of at least a portion of a pattern of a first mask to be formed through a first lithographic process of the plural lithographic processes.
In one preferred form of these aspects of the present invention, the method includes (i) a first process for etching a portion of a base material not covered by the first mask to a predetermined depth, (ii) a second process for forming a second mask so that it covers a particular region of a portion of the base material not covered by the first mask and also that it overlaps with the first mask and, subsequently, for etching the base material to a predetermined depth by using the first and second masks as an etching mask, and (iii) a third process in which, after the second mask is separated, the second process is repeated as required and, after the second mask is separated, a third mask is used to cover the portion not covered by the first mask, in which a fourth mask is formed so that an end of a pattern of the fourth mask lines at an end portion of the pattern of the first mask while an opposite end of the pattern of the fourth mask overlaps with the pattern of the third mask, and in which, after the fourth mask is formed, an exposed portion of the first mask is removed by etching so that the base material is exposed and, subsequently, the exposed portion of the base material is etched to a predetermined depth, wherein the third process is repeated at least once, as required, after the third and fourth masks are separated.
In another preferred form of these aspects of the present invention, the method includes (i) a first process for etching a portion of a base material not covered by the first mask to a predetermined depth, (ii) a second process for forming a second mask so that it covers a particular region of a portion of the base material not covered by the first mask and also that it overlaps with the first mask and, subsequently, for etching the base material to a predetermined depth by using the first and second masks as an etching mask, and (iii) a third process in which the second process is repeated as required after the second mask is separated and, after the second mask is separated, a third mask is formed so that it covers the portion not covered by the first mask and that an end of a pattern of the third mask lies at an end portion of the pattern of the first mask while an opposite end of the pattern of the third mask lies on the first mask, and in which, after the third mask is formed, an exposed portion of the first mask is etched, wherein the third process is repeated as required after the third mask is separated.
In a further preferred form of these aspects of the present invention, the method includes (i) a first process for forming a second mask so that it covers a particular region of a portion of a base material not covered by the first mask and, subsequently, for etching the base material to a predetermined depth by using the first and second masks as an etching mask to remove the second mask, (ii) a second process for repeating the first process, at least once, so that the portion of the base material not covered by the first mask is etched to a predetermined depth, and (iii) a third process in which a third mask is used to cover the portion not covered by the first mask, in which a fourth mask is formed so that an end of the fourth mask lies on the first mask while an opposite end of the pattern of the fourth mask overlaps with the third mask, and in which, after the fourth mask is formed, an exposed portion of the first mask is removed by etching so that the base material is exposed and, subsequently, the exposed portion of the base material is etched to a predetermined depth, wherein the third process is repeated as required after the third mask is separated.
In a still further preferred form of these aspects of the present invention, the method includes (i) a first process for forming a second mask so that it covers a particular region of a portion of a base material not covered by the first mask and, subsequently, for etching the base material to a predetermined depth by using the first and second masks as an etching mask to remove the second mask, (ii) a second process for repeating the first process at least once so that the portion of the base material not covered by the second mask is etched to a predetermined depth, and (iii) a third process in which a third mask is formed, after the second mask is separated, so that the third mask covers at least the portion not covered by the first mask and also that an end of a pattern of the third mask lies at an end of a pattern of the first masks while an opposite end of the pattern of the third mask lies on the first mask, and in which, after the third mask is formed, an exposed portion of the first mask is removed by etching so that the base material is exposed and, subsequently, the exposed portion of the base material is etched to a predetermined depth, wherein the third process is repeated as required after the third mask is separated.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.